An irate little man that pops out of your butthole and screams at you to stop grunting and wiping your ass so hard. He also punishes you for eating Mcdonalds everyday and drinking a case of beer every night.
But since most people get them it's probably just from having an enormous network of pulsating veins at the end of your poopshoot.

Aiiiiieee !!! The little man is back. He screamed at me for sneezing while I was taking a dump.